The present invention is directed to a silicon on insulator (SOI) structure having a low defect density device layer. More specifically, the present invention is directed to a SOI structure wherein the device layer is derived from a single crystal silicon wafer which is substantially free of agglomerated intrinsic point defects.
Additionally, the present invention is directed to a SOI structure having a single crystal silicon handle wafer which is capable of forming an ideal, non-uniform depth distribution of oxygen precipitates, upon being subjected to the heat treatment cycles of essentially any arbitrary electronic device manufacturing process.
A SOI structure generally comprises a handle wafer, a device layer, and an insulating film, (typically an oxide layer) between the handle wafer and the device layer. Generally, the device layer is between 0.5 and 20 micrometers thick. Such a wafer may be prepared using various techniques known in the art. For example, wafer thinning techniques may be used, often referred to as back etch SOI (i.e., BESOI), wherein a silicon wafer is bound to the handle wafer and then slowly etched away until only a thin layer of silicon on the handle wafer remains. (See, e.g., U.S. Pat. No. 5,189,500). Alternatively, a single wafer may be used wherein molecular oxygen ions (O.sub.2.sup.+) or atomic oxygen ions (O.sup.+) are implanted below the surface of the wafer to form an oxide layer. This process is generally referred to as SIMOX (i.e., separation by implantation of oxygen; see, e.g., U.S. Pat. No. 5,436,175 and Plasma Immersion Ion Implantation For Semiconductor Processing, Materials Chemistry and Physics 46 (1996) 132-139). Such a process is considered advantageous because it acts to reduce the number of silicon wafers which are consumed, as compared to the more conventional wafer thinning processes, in the preparation of a SOI structure.
SOI structures may be prepared from silicon wafers sliced from single crystal silicon ingots grown in accordance with the Czochralski method. In recent years, it has been recognized that a number of defects in single crystal silicon form during the growth process as the crystal cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects, which are known as vacancies and self-interstitials. Silicon crystals grown from a melt typically contain an excess of one or the other type of intrinsic point defect, either crystal lattice vacancies or silicon self-interstitials. It has been suggested that the type and initial concentration of these point defects in the silicon are determined at the time of solidification and, if these concentrations reach a level of critical supersaturation in the system and the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur. Agglomerated intrinsic point defects in silicon can severely impact the yield potential of the material in the production of complex and highly integrated circuits, such as those utilizing SOI structures.
Vacancy-type defects are recognized to be the origin of such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques such as Scanning Infrared Microscopy and Laser Scanning Tomography. Also present in regions of excess vacancies are defects which act as the nuclei for ring oxidation induced stacking faults (OISF). It is speculated that this particular defect is a high temperature nucleated oxygen agglomerate catalyzed by the presence of excess vacancies.
In addition to the above-mentioned vacancy-type defects, it is also believed that agglomerated vacancy defects, or voids, may be the cause of "HF defects" (i.e., metal precipitation defects). HF defects are, like these other vacancy-type defects, considered to be a critical problem with current SOI technology.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
Agglomerated intrinsic point defects can create performance problems for SOI substrates if silicon wafers containing such defects are utilized as the source of the device layer. Performance problems may also result from metallic contaminants present in the handle wafer portion of the SOI structure. During the heat treatments employed by the SOI process, metallic contaminants, present in the handle wafer as a result of cleaning and handling of the SOI structure, may migrate through the silicon matrix until the oxide layer, present between the handle wafer and the device layer, is reached. Although generally speaking these impurities may not pass through the oxide layer and into the device layer, the oxide layer is a preferential site for the precipitation of these impurities. This precipitation acts to disrupt the oxide layer and interfere with the performance of the SOI device.
Accordingly, a need continues to exist for a SOI substrate which contains a device layer which is substantially free of agglomerated intrinsic point defects. Additionally, a need continues to exist for a SOI substrate which contains a handle wafer capable of inhibiting the precipitation of metallic impurities at or near the oxide layer/silicon interface.